Engineering New Zealand has been hearing from engineers and councils that demonstrating durability under B2 remains a problem, so we’ve been looking for solutions.

B2 is a part of the Building Code that specifies durability of elements of structures. For example, a structure that has a design life of 50 years should be constructed of materials with a tested life of at least 50 years. This has become an issue between engineers and councils, typically around the life of exposed steelwork.

Steel can be protected from corrosion and degradation either by sheltering it from the elements (and ensuring it remains within a weathertight building envelope) or by coating it with a corrosion-inhibiting substance; for example, by galvanising or alternative coatings. These coatings are generally determined by verification methods in codes such as NZS 3404:1997 and AS/NZS 2312. Alternatively, if the steelwork is protected by remaining within a weathertight envelope, it is accepted that it will last for the life of the structure because corrosion should be minimal.

The problem with coatings is they protect the exposed steel only for 25 years at the most. Obviously for a building with a design life of 50 years, this poses an issue for the remaining 25 years. In 2018, NZS 3404.1 2009 was superseded by SNZ TS 3404:2018, which provides calculations in section 2.5 around how to calculate steel loss once the protective coating has gone. This lets you oversize steel members to allow for potential corrosion until the end of the design life of the building. Unfortunately, especially near the sea, 45 years is often the maximum design life achievable due to the thickness of steel available.

This can put engineers in a difficult position. However, the Ministry of Business, Innovation and Employment (MBIE) has recently issued a helpful determination (2019/30) on durability issues. 

The determination concerns an authority’s refusal to issue a building consent to carry out invasive investigation and reinstatement work on the potential corrosion of concealed structural steel elements in an apartment complex because the engineer’s producer statement did not include the structural durability of the proposed work. The design engineer had provided drawings, photographs and specifications for the proposed work, supported by a PS1 and PS2, but had not included B2 in the Building Code Clause(s) section of the producer statements, only B1/VM1. They had also followed Engineering New Zealand’s instructions for providing supplementary letters and calculations to demonstrate how compliance with B2 was achieved, referencing relevant industry standards for concrete and steel structures and the environmental conditions of the site.

The determination notes the task of an authority under section 14F(a)(i) of the Building Act 2004 is to properly assess the adequacy of all information submitted for a consent application, regardless of the format of that information. It says the engineer used every available verification method available (NZS 3404:1997 and AS/NZS 2312) to ensure B2 durability. It said for the council to demand further evidence from that engineer was unreasonable. MBIE found that it was acceptable for the engineer to specify compliance through B1/VM1 provided the appropriate clauses and standards are shown.

We have been engaging with a number of councils and council checkers on the best way for engineers to show compliance. If steelwork cannot be shown to last for the full life of the building, then one way is to provide a maintenance plan for the steel work. This can be done in conjunction with SNZ TS 3404:2018, section 1.7. When doing so, ensure the maintenance schedule is clearly shown on drawings and in calculations. Putting a reference to the maintenance schedule in your design features report is a good idea, to help council checkers easily find the information. You should also ensure the steelwork in question can realistically be inspected and maintained as required.

As referenced in the determination, Engineering New Zealand has developed example letters you can use to demonstrate B2 durability, as well as an advisory on B2. You'll find these in the members' resource area under the heading 'B2 – Durability'.